(1) Field of the Invention
This invention relates to the art of non-invasive measurement of the pressure inside pipes or other vessels. More specifically, this invention relates to a fiber optic interferometer used for detecting changes in the outer circumference of a pipe and thereby predicting the pressure inside that pipe.
(2) Description of the Known Art
It is often necessary or desirable to monitor the pressure of a fluid or gas flowing in an expansible pipe without destroying the integrity of the pipe. Known non-invasive methods include strain gauge sensors and piezoelectric sensors.
With strain gauge sensors, the pressure is measured by measuring the pipe wall expansion using a plurality of sensors positioned circumferentially around the pipe. The sensors used are typically strain gauges or force gauges. One variety of this class of sensors involves using two rigid beams which are clamped around the pipe perpendicular with its length. The beams are fixed spatially at one end, while a sensitive strain gauge is attached between the beams at the opposite end. In this manner, the beam lengths mechanically amplify any diametric size variations in the pipe such that they can be detected by the strain gauge.
A primary disadvantage of the strain gauge type sensors is that they sense local, diametric expansion rather than circumferential expansion. This makes them particularly vulnerable to errors introduced by structural vibrations of the pipe wall. The strain gauge sensors are sensitive to flexural wave, compressional wave, and torsional wave motions.
The second class of sensors employs piezoelectric transducers. A strip of piezoelectric film is wound around a pipe one or more times. Expansion of the pipe wall then results in a flexure of the piezoelectric element which causes the element to generate an electrical charge. The magnitude of this charge can then be related to changes in the pipe's internal pressure.
Since the piezoelectric film extends around the entire circumference of the pipe wall, the resultant charge output represents a circumferential average of the charges resulting from the local pipe wall deformations. Thus, the piezoelectric sensor has the advantage over the mechanical sensor in more completely discriminating against components of the pipe wall response resulting from axial and circumferential waves propagating thereon. However, since the piezoelectric element is sensitive to strains in all directions, it is susceptible to strain errors resulting from torsional and compressional pipe wall response.
The piezoelectric film sensor has several further disadvantages. First, the sensor responds only to dynamic or changing pressures. This is because the charge "bleeds off" of the piezoelectric device, making it unable to measure static pressures. Second, the piezoelectric film is especially sensitive to electromagnetic radiation, such that the desired signal may be masked by electrical noise. This sensitivity also requires that the piezoelectric film be carefully insulated from the pipe wall if the pipe is electrically conductive. Third, the piezoelectric material is temperature sensitive, and there is no simple method for correcting for its temperature variations. Fourth, piezoelectric material cannot be used in environments where the temperature is greater than 200.degree. F.
It is towards overcoming the limitations of these known devices that the present invention is directed.